In small electric appliances, an oscillating direct drive, such as an oscillating electric motor, can be provided to drive mechanical parts capable of oscillation. Such types of drives are used, for example, in electric razors or electric toothbrushes whose working amplitude is generated without gearing. The mechanical parts capable of oscillation are primarily the rotors of the oscillating electric motor, the drive shaft, and optionally a component coupled to the drive shaft, for example a replaceable brush. In order to achieve a high level of efficiency, it is desirable for the oscillating electric motor to be operated with alternating current whose frequency takes into consideration the resonance frequency of the oscillating mechanical parts of the small appliance.
Determination of the resonance condition to drive oscillation-capable mechanical parts via the power consumption of the electric motor or the working amplitude of the oscillating parts is known. The disadvantage of this is that quantitative measuring processes must be carried out, for example, to determine the minimum of the current or the maximum of the working amplitude. To that end, measuring equipment must be provided for carrying out the measuring processes.
From German published patent application DE 10 2004 029 684 A1 is known the determination of the resonance condition to drive oscillation-capable mechanical parts via an analysis of a die-out oscillation process of the mechanical parts over time. To do this, the electric motor is switched on for a short period of time so that it can execute a few oscillation movements and the oscillation-capable parts can achieve a state of resonance. The electric motor is then switched off so that the oscillation movements of the electric motor die out. During this process, the electric motor generates induced voltage corresponding to the oscillation movements for a brief period. The frequency of the induced voltage is measured and then considered in the repeated electric actuation of the electric motor. Quantitative measuring processes are also used here, which have the previously mentioned disadvantages. A further disadvantage exists in that the results of the analysis of the die-out oscillation process depend on the damping properties of the oscillation-capable mechanical components.
If the damping properties are not known or if the damping properties change over time, this can negatively affect the efficiency of the system.